Ichneumonidae is a family within the insect order Hymenoptera. Insects in this family are commonly called ichneumon wasps. Less exact terms are ichneumon flies (they are not closely related to true flies), or scorpion wasps due to the extreme lengthening and curving of the abdomen (scorpions are not insects).
Simply but ambiguously these insects are commonly called "ichneumons", which is also a term for the Egyptian Mongoose (Herpestes ichneumon); ichneumonids is often encountered as a less ambiguous alternative. Ichneumon wasps are important parasitoids of other insects. Common hosts are larvae and pupae of Coleoptera, Hymenoptera, and Lepidoptera.
There are over 60,000 species worldwide, and approximately 3,000 in North America - more than any other Hymenoptera family. The distribution of Ichneumonidae is one of the most notable exceptions to the common latitudinal gradient in species diversity because it shows greater speciation at high latitudes than at low latitudes.
Ichneumon wasps differ from typical wasps, which sting in defense (Aculeata: Vespoidea and Apoidea), in that the antennae have more segments; typically 16 or more, whereas the others have 13 or fewer. An ichneumon wasp's abdomen is characteristically very elongated, unlike in their relatives the braconids. This lengthened section may also be segmented.
Female ichneumon wasps frequently exhibit an ovipositor longer than their body. Ovipositors and stingers are homologous structures; some Ichneumons inject venom along with the egg, but they do not use the ovipositor as a stinger, per se, except in the subfamily Ophioninae. Stingers in aculeate Hymenoptera - which like Ichneumonidae belong to the Apocrita - are used exclusively for defense; they cannot be used as egg-laying equipment. Males do not possess stingers or ovipositors in either lineage.
Description taken from Wikipedia: http://en.wikipedia.org/wiki/Ichneumonidae